In FIRST Robotics Competition robots, drivetrains consist of all the components needed to make the robot move around the game field. Drivetrains are a crucial part of the robot and are often one of the first parts of the robot that a team begins designing and iterating upon.
Most drivetrains will use motors connected to gearboxes to transmit motion to wheels with belts or chain. Some teams will incorporate parts such as pneumatics and servos into their drivetrains as well, allowing for shifting of gears and changing what wheels are in contact with the ground at certain points in the game.
Drivetrains have various forms, each having its own pros and cons- the optimal drivetrain for a team is one that finds the correct balance of speed, maneuverability, and power to fit within that team's specific strategy.

Types of Drivetrains

West Coast Drive

West Coast Drive is generally characterized by having six, eight, or ten wheels that are cantilevered. Further, one wheel on each side is directly driven, while the rest are linked via chain. To help improve maneuverability, omni wheels can be used on the front or back, so wheels slide across the floor while turning. When using six or more wheels, lowering the middle wheel of the drivetrain can cause less friction because not all wheels are touching the floor at one time.
See REV's West Coast Drive built using ION products here:

ION West Coast Drivetrain - Onshape File

Swerve Drive

Swerve drivetrain uses standard wheels mounted on their own pivoting mechanisms. Most swerve drivetrains require two motors per wheel; one to rotate the wheel, and one to control the direction the wheel points. Your robot can move in any direction because the drive wheels pivot without changing the orientation of the drivetrain, but this type of drivetrain is very technically complex. There are multiple types of swerve drive, including crab drive and 2+2 configuration.
Swerve Drives offer a huge increase in maneuverability, at the cost of increased build complexity and increased resource costs, including weight, space, capital, and time.
See REV's Swerve Drive built using ION products, including the MAXSwerve Module here:

ION 3in MAXSwerve Drivetrain - Onshape File

Other Drivetrains

Some other common drivetrains used in FIRST include:
  • Two Wheel Drive (also known as "casterbot")
  • Four Wheel Drive
  • Six Wheel Drive variations that are not specifically West Coast Drive
  • Car Drive
  • H-Drive
  • Holonomic Drive
  • Mecanum Drive
All of these systems have their own advantages and disadvantages that should be discussed by your team. This is by no means a comprehensive list, either!
As teams learn, adapt, and iterate on each year's unique challenges, new drivetrains are created annually to meet the criteria needed to succeed and make it all the way to Einstein!